Downbeat reports from Alcoa and Chevron at the start of corporate earnings season pulled stock indexes lower for a third straight day Wednesday. The Dow Jones industrial average slumped more than 128 points, its steepest loss since late June.

Alcoa, the aluminum producer, beat Wall Street's earnings estimates on Tuesday night but said it expects a slowdown in China to weaken demand for aluminum. Its stock fell 42 cents Wednesday to $8.71. The company is often used as a bellwether for the global economy.

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Chevron, the country's second-largest oil company, warned late Tuesday that slumping oil prices and production would cause earnings to be "substantially lower." It blamed Hurricane Isaac for disrupting production at a Mississippi refinery.

On Tuesday, the Supreme Court also refused to block a $19 billion judgment levied against Chevron by an Ecuadorean court for polluting the Amazon.

The Dow fell 128.56 points to close at 13,344.97. The Standard & Poor's 500 index fell 0.62 percent to 1,432.56. The Nasdaq composite index fell 0.43 percent to 3,051.78.

Alcoa and Chevron's results were an unpromising start to the third-quarter earnings parade, said JJ Kinahan, chief derivatives strategist at Ameritrade.